A reserve zirconium phosphate module for continuing dialysis in the event the capacity of the original zirconium phosphate module is exceeded. The sorbent cartridge can have a sensor for detecting when the capacity of the zirconium phosphate material has been exceeded, and a valve assembly for diverting the flow of spent dialysate into the reserve module when needed. Any of the modules of the sorbent cartridge can be reusable and the sorbent materials therein recharged.